The present invention relates to a process for coating plastic substrates with a transparent layer compact that is not sensitive to scratches and abrasion and resistant to atmospheric influence by using a plasma burning in a vacuum chamber at some distance from the substrate.
It is known how to protect plastic objects by depositing a varnish-like layer, which must be transparent itself if the plastic is transparent. In the latter case acrylate, polyurethane, siloxane, ormocer, or nanocer coatings have been used with success, all of which offer protection in varying degrees against scratches and abrasion and UV rays. All of these coatings have in common that they are applied with the standard techniques for applying varnish. A series of disadvantages, which occur generally in solvent emissions and problems with the consistency of the coating, with drying, or with dust, result from this. The solution to these problems and the coating material itself are associated with considerable costs.
An alternative is to apply inorganic protective layers, for example, of silica, with the help of vapor deposition in vacuum. Thus, in WO 97/23661 a high-speed vaporization process from an oxide-former is proposed, whereby oxygen or nitrogen and additional gas-forming monomers are channeled into the vaporization zone and this mixture passes through a high-density plasma zone near the substrate to be coated.
Another very effective process resides in applying an optically transparent protective layer by using plasma chemical vapor deposition (PCVD). In this case silicon compounds, usually in the form of silanes (DE 26 50 048 and EP 0 252 870), silazanes, or siloxazanesxe2x80x94often hexamethyldisiloxane (HMDSO)xe2x80x94are converted into a layer, i.e., polymerized, usually with the addition of oxygen in a low-temperature plasma. Moreover, in U.S. Pat. No. 5,298,587 a layer produced with PCVD of the general composition SiO:1.8-2.4C0.3-1.0H0.7-4.0 is proposed as protection against abrasion for various plastic parts consisting of diverse polycarbonates. As a further development it was proposed in DD 238 630 to add nitrogen and a carrier gas, for instance, argon, to an organosilicon compound. In EP 0 317 134 an object coated with a transparent, abrasion-resistant layer is described that is produced through plasma CVD of an unsaturated hydrocarbon together with a silane or fluorosilane and an oxygen source, whereby the composition of the layer is homogeneous to a great extent.
By carrying out the process appropriately, the composition of this layer can be varied in a wide range, whereby layers that are rich in amounts of hydrocarbons are distinguished by hardness and resistance to abrasion and by a certain plasticity that is low in its proportion of hydrocarbons, and as a result the layers are like silicon. This can be used to apply layers, whose hardness increases as it goes toward the surface, to plastic objects with a relatively low hardness (DE 34 13 019, U.S. Pat. No. 4,927,704, U.S. Pat. No. 5,051,308, EP 0 267 679, WO 97/13803). In so doing the differing thermal coefficients of expansion for plastic material and coating, which express themselves in the layer""s improved adhesion over time, can be balanced out.
In DE 42 38 279 it is proposed that plastic objects be coated with a transparent, adhesive, and scratch-resistant layer, which also provides a good barrier effect against water, chemicals, and UV rays. In this connection a coating consisting of at least silicon, oxygen, and carbon inside at least three depths of field is standardized for different chemical compositions, whereby the silicon and oxygen content generally rises with the layer thickness and the carbon content generally falls with the layer thickness. However, experience has shown that a sufficient UV-protective effect cannot be obtained.
There nevertheless remains the problem that with SiOxCyHz or SiNxCyHz layers produced with PCVD, UV rays are transmitted regardless of the exact stoichiometry of the layers. This is especially disadvantageous when the plastic objects consist of especially UV-sensitive plastics such as polycarbonate (PC), for the surface of these plastics can be damagedxe2x80x94in spite of added UV absorbersxe2x80x94in such a way that a coated PCVD protective layer thereon detaches itself from the surface. There has therefore been no dearth of attempts to make transparent PCVD protective layers impenetrable to UV rays. For this purpose according to U.S. Pat. No. 5,156,882 a PCVD scratch-protection layer is combined with an additional UV-absorbing PVCD layer made of ZnO, TiO2, CeO2, or V2O5. It is, however, disadvantageous that such an additional layer can attract negative attention with its colored appearance because it has a higher refractive index than the plastic and the scratch-protection layer. This effect is lost with layer thicknesses larger than about 1 xcexcm, but coating of such a thickness is not economical with the relatively low vapor-deposition speeds of these layers.
In EP 92 400 581 and EP 0 502 790 PCVD multi-layer systems are described that have a UV interference trap effect. Also in this case differences in the layer thickness are conspicuously visible and thus critical.
In U.S. Pat. No. 5,270,854 an optical filter coating consisting of a dye, which is deposited in a diluted, inorganic layer (preferably as vacuum-vaporized SiO2), is proposed, whereby PC is used as a substrate material and is precoated with a barrier layer made of plasma-polymerized HMDSO and a protective layer made of a silicon polymer.
In accordance with JP 6025488, JP 1301723, and DE 195 22 865 an organic UV absorber is vaporized during a PCVD process for depositing a scratch-protection layer and is built into the layer. Because the most economical PCVD processes are based on a high fragmentation level of the starting substances, the danger is great that when the UV absorbers are deposited at high speeds, fragmentation will occur that is too great and thus ineffective.
On the other hand, efforts have also been made to produce UV-absorbing substances from acetone, 2-butanon, or benzene during the deposition of a PCVD scratch-protection layer in plasma. UV weathering tests nevertheless do not show an improvement in the UV stability.
Finally, EP 0 285 870 provides a solution in which the PC parts are first precoated with an intermediate layer consisting of an silicon-organic resin that contains a UV-protective compound and then are coated with an abrasion-protection outer layer, which is produced by using plasma CVD.
The surface of industrial plastics is usually sensitive to mechanical stress such as abrasion or scratching. When being used outside, the duration of use is often limited by deficient UV resistance because the material turns yellow and/or cloudy. This is especially disadvantageous with plastic objects whose function is associated with the transparency of the materials, e.g., with helmet-mounted sights, covering panes of vehicle headlights, instrumentation panels of automobiles, or plastic panels on buildings.
Therefore an object of the present invention is to create a transparent protective coating for transparent plastics such as polycarbonate (PC), polymethylmethacrylate (PMMA), or CR 39 that not only has an improved resistance to scratching and abrasion, but also an improved resistance to light rays and the influence of temperature and moisture, which, for example, is characteristic in outside weathering, and thus avoids the disadvantages of the wet deposition technique.